GB2301949A - Mounting SiC rectifiers in an AC generator - Google Patents

Abstract

To provide a low-loss vehicular AC generator (200) in which a rectifier (19) is mounted on a front side of a transversely-mounted engine (100) with a front exhaust pipe (101) layout and fixed to an exhaust pipe-side end wall of the housing (1, 2) of the generator (200), a three-phase full wave rectifier (19) employs MOS power transistors (19a-19f) formed of monocrystalline SiC as semiconductor rectifying devices. The rectifier (19) is fixed to an exhaust pipe-side end wall of a generator housing (1, 2). This construction considerably reduces the heat generation of the rectifier (19) and enhances its heat resistance, so that the disposal of the rectifier (19) on the exhaust pipe-side end wall of the generator housing (1, 2), which is a severe thermal environment due to radiant heat from the exhaust pipe (101), will not degrade the reliability of the rectifier (19).

Description

VEHICULAR AC GENERATOR EMPLOYING BEAT-RESISTANT HOUSING-WOUNTED SiC RECTIFIERS The present application is related to and claims priority from Japanese Patent Application No. Hei. 7-139739, incorporated herein by reference.

The present invention relates to a vehicular alternating current generator in which a rectifier is fixed to a housing.

Along with a recent trend of engine size reduction and power increase and engine compartment size reduction, many front engine-front wheel drive vehicle models employ a so-called front exhaust pipe-transverse mount engine layout in which the engine is transversely mounted with the exhaust pipe extending from the front side of the engine. This layout allows for installation of a high power engine without requiring an increase in size of the engine compartment.

In this front exhaust pipe-transverse mount engine layout, the generator is normally fixed to a front side of the engine.

This front generator arrangement is chosen because the generator will be readily cooled by air currents caused by the traveling of the vehicle and air currents generated by a cooling fan -1 disposed near a front end of the engine compartment. In addition, since the generator is pulley-driven, installation of the generator over, under or in the rear of the engine would face problems, such as limited space.

In conventional vehicular AC generators, the rectifier is fixed to the housing of the generator. This arrangement provides advantages in that the housing can also be used as a heat sink and a ground terminal (body grounding) utilizing the large heat capacity, good thermal conductivity and good electric conductivity of the housing. In addition, since the connecting distance between the generator and the armature winding is short, the wiring therebetween is easy and power losses due thereto will be reduced. According to the conventional art, the rectifier is normally fixed to a side wall of the generator housing remote from the pulley (also referred to as "rear end wall"). Other fixation arrangements would cause problems.If the rectifier were fixed to the pulley-side wall of the generator housing, the protruding rectifier would interfere with the assembly operation of setting a belt on the pulley. If the rectifier were fixed to the peripheral side wall of the generator housing, the increased radial dimension would impede the assembly operation of mounting and fixing the generator to the engine.

Japanese Patent Application Laid-Open No. Sho. 4-138030 discloses that a MOS power transistor is used as a semiconductor rectifying device of the rectifier. Such a MOS power transistor usually employs a vertical MOS power transistor construction in which N-type silicon substrate is used as one of the main electrodes of the MOS power transistor and the other main electrode is constructed by forming an N+ type region in a surface portion of the P well region formed in a surface layer of the chip.

With the arrangement in which a vehicular AC generator is disposed in front of a transversely-mounted engine with a front exhaust pipe and the rectifier is fixed to the rear end wall of the generator housing remote from the pulley, however, there are problems in that the rectifier fixed to the end wall of the generator housing remote from the pulley comes close to the exhaust pipe, which becomes hot and heats the rectifier to high temperatures by heat radiation, despite the advantages in that the front-to-rear or top-to-bottom dimension of the engine room required for installation of the engine and the generator can be reduced and that the generator can be effectively exposed to the air currents generated by the traveling of the vehicle and the operation of the cooling fan. The conventional highest allowable temperature of the rectifier is approximately 180 degrees Celsius.The temperature of the rectifier must be controlled so as not to exceed 180 degrees Celsius even under the severest driving conditions. It is a usual practice to protect and shield the rectifier fixed to the rear end wall remote from the pulley from radiant heat by covering the rectifier with an end cover fixed to the rear end wall. However, since the end cover must be formed of a metal (normally, aluminum) to resist such high temperatures, the end cover is heated by radiant heat and, in turn, heats the rectifier and the housing by heat radiation or conduction.

In short, the problem of overheating of the rectifier arises with employment of the space-minimum arrangement in which a generator is disposed in front of a transversely-mounted engine with the front exhaust pipe layout and the rectifier is fixed to the rear end wall of the generator housing remote from the pulley. A heat insulating structure may be added between the exhaust pipe and the end cover. However, this will cause further problems of increased numbers of component parts and assembly steps, and complicated maintenance.

Size increase of the rectifier cooling fan may also be considered to control the rectifier temperature so as not to exceed the highest allowable temperature (about 180 degrees Celsius). The increased cooling air flow from the fan for cooling the interior of the generator would improve the rectifier cooling characteristics. However, this would result in size increase of the rectifier, temperature increase of the armature winding and the field winding due to increased ventilation resistance, and increases of the heat radiated to the rectifier cooling fan.

The present invention is intended to solve the above problems. An object of the invention is to achieve high reliability of the rectifier in an arrangement where a generator is disposed in front of a transversely-mounted engine with a front exhaust pipe and the rectifier is fixed to the end wall of the generator housing remote from the pulley, without taking a measure that would cause a secondary problem, such as addition of a heat insulating structure or size increase of the cooling fan as described above.

The present inventors have considered employing a rectifier comprising a MOS power transistor as the semiconductor rectifying device to achieve the above object. In contrast with a rectifier employing a diode as the semiconductor rectifying device, the rectifier using a MOS power transistor suffers no heat generated by forward junction loss and exhibits only resistance loss.

Thus, the MOS power transistor rectifier is likely to achieve temperature reduction. However, the present inventors' analysis has revealed the following problems with above-mentioned conventional MOS power transistor three-phase full wave rectifiers.

First, since large amounts of electromagnetic energy are accumulated in the three-phase armature winding and the field coils of a vehicular AC generator, each semiconductor power device of the three-phase full wave rectifier needs to withstand at least twenty times the battery voltage, that is, the rectified output voltage of the three-phase full wave rectifier, for example, at least 300 V, as countermeasures for accidental instant release of the accumulated energy. Furthermore, today's increased vehicle-installed electrical loads (including a defroster heater) require current as great as or greater than 100 A. MOS power transistors having such a high voltage-withstanding and great current-outputting structure suffer substantially as much power loss as diodes, thus eliminating the advantages of substituting the diodes with more complicated MOS power transistors.

The above-stated problems of MOS power transistor type three-phase full wave rectifiers will be discussed in detail with reference to FIGS. 4 and 5. FIG. 4 shows an inverter circuit illustrating a one-phase portion of a MOS power transistor type three-phase full wave rectifier. FIG. 5 shows an example cross section of a typical MOS power transistor.

In the inverter circuit having N channel MOS power transistors shown in FIG. 4, the drain electrode D of a MOS power transistor 101 at the high side and the source electrode S of a MOS power transistor 102 at the low side are connected to a single-phase output terminal of a three-phase AC generator (not shown), and the drain electrode D of the low-side MOS power transistor 102 connected to the lower potential terminal of a battery (not shown). The source electrode S of the high-side MOS power transistor 101 is connected to the higher potential terminal of the battery. The direction of electron transfer and the direction of charging current during charging are opposite to each other.

Furthermore, in each of the MOS power transistors 101, 102, a source-side parasitic diode Ds and a drain-side parasitic diode Dd are theoretically formed between the source electrode S and the P type well region and between the drain electrode D and the P type well region as indicated in the drawing.

To provide an electric potential for the P type well (for example, a region 103 in FIG. 5), that is, a given conduction type semiconductor region having an inverted channel in its surface portion, the P type well region is normally connected to either one of the opposite conduction type source electrode S (for example, a region 106 in FIG. 5) and drain electrode D (for example, a region 104 in FIG. 5) that are connected to each other by the inverted channel. If the inverter circuit shown is used as a single-phase circuit of the three-phase full wave rectifier, it is necessary to connect the P type well (for example, a region 103 in FIG. 5) to the drain electrode D (for example, the region 104 in FIG. 5), that is, to short-circuit the drain-side parasitic diode Dd.

In a vehicular AC generator, if the P type well region (for example, 103 in FIG. 5) and the source electrode S (for example, 106 in FIG. 5) are connected and the source-side parasitic diode Ds is short-circuited, reverse current will flow through the drain-side parasitic diode Dd when the generated voltage to the drain electrode D of the high-side MOS power transistor becomes lower than the battery voltage. Reverse current will also flow through the drain-side parasitic diodes Dd when the generated voltage to the source electrode S of low-side MOS power transistor rises above the electric potential (ground electric potential) of the battery lower potential terminal. To prevent reverse current through the parasitic diodes Dd, the P type well regions 103 need to be connected to the drain electrodes so that the reverse current flows through the source-side parasitic diodes Ds.In short, the P type well regions of the MOS power transistors (for example, 103 in FIG. 5) used in a vehicular AC generator need to be connected to the drain electrodes D.

However, in the conventional vertical channel MOS power transistor construction shown in FIG. 5, the voltage withstanding characteristic must be achieved by short-circuiting the P type well region 103 and the surface N+ type region 104 and expanding a depletion layer 107 formed at the PN junction between the P type well region 103 and an N type epitaxial voltage withstanding layer 105 into the N type epitaxial voltage withstanding layer 105.

Thus, if the vehicular AC generator is constructed using conventional MOS power transistors as shown in FIG. 5, it is necessary to form the N+ type substrate 106 as the source region and the N+ type region 104 as the drain region. However, this design includes serial connection of a large source parasitic resistance Rs between the substantial source end S' and the source electrode.

If the threshold voltage Vt is ignored for simplified discussion, the drain saturation current Idsat of the MOS transistor is expressed as follows: Idsat = K(Vg - Vs')2 = K(DVgs - IdsateRs)2, where K is a constant of proportion, (Vgs is gate-source voltage (Vg-Vs), Vg is a gate voltage, and Vs' = Vs+IdsatRs is the electric potential of the substantial source end S'. This expression means that the drain saturation current (the maximum current occurring when a predetermined gate voltage is applied) Idsat is reduced corresponding to a reduction of the gate voltage Vg by IdsatRs. A change of the threshold voltage Vt caused by substrate effects is also ignored in addition to above.

For example, if gate voltage +20 V, the source (battery) electric potential is +12 V, the current is 100 A, and the source parasitic resistance Rs is 0.05 , then the actual source potential Vs is 17 V, and the channel current is reduced to 9/64 compared with the case where Rs is 0. As understood, a small increase of the source parasitic resistance Rs drastically reduces the channel current. Hereinafter, this current reducing effect, in other words, the channel resistance increasing effect, will be referred to as "source resistance feedback effect".

Although the above expression is for the drain current saturation region, the drain non-saturation current also decreases corresponding to an increase of Rs in the non-saturation region. A drain current increase means a channel resistance increase. An increase of the source parasitic resistance Rs causes power loss and increases the channel resistance to also cause power loss. Thus, it should be understood that an increase of the source parasitic resistance Rs cause a large power loss and heat generation in total.

The source parasitic resistance Rs of the MOS power transistor shown in FIG. 5 can be reduced by reducing the thickness of the N type voltage withstanding layer 105. However, it is difficult to reduce the thickness of the N-type voltage withstanding layer 105 because the MOS power transistors for the vehicular AC generator must withstand a voltage as high as 300 V.

In a normal silicon MOS power transistor, the silicon yield field strength is about 30 V. To achieve a 300 V voltage withstanding characteristic only using the N type voltage withstanding layer 105, the layer 105 needs to have a thickness of 10 pm, assuming that the field strength is constant. If the field strength inside the N type voltage withstanding layer 105, the layer 104 must have a thickness of at least about 20 um and an impurity concentration of about 1 x 1015 atoms/cm3 or less, in order to achieve the 300 V withstanding characteristic.

Formation of an N type voltage withstanding layer 105 having such thickness and impurity concentration in order to achieve a sufficient voltage withstanding characteristic will cause an increase of the source parasitic resistance Rs and a consequent resistance loss and a reduction of the drain current (a drastic increase of the channel resistance) as described above.

Therefore, the MOS power transistor three-phase full wave rectifier disclosed in the above-described Japanese Laid-Open Patent Publication is theoretically not able to surpass PN-junction diode three-phase full wave rectifiers in application to vehicular AC generators (that is, applications to reactance loads) and would provide only the drawbacks of complication of both structure and control. The MOS power transistor three-phase full wave rectifier thus fails to provide practical merits.

Another structure can be considered, in which N+ type region 104 and the N+ type substrate 106 of the MOS power transistor construction shown in FIG. 5 are formed as the source electrode and the drain electrode, respectively, and the P type well region 103 and the N+ type drain region 106 are short-circuited as indicated in FIG. 4. With this structure, however, it : extremely difficult to achieve a 300 V withstanding characteristic between the N+ type region (source electrode) 104 and the P type well region 103 and a sufficient voltage withstanding characteristic between the gate electrode and the P type well region 103 and between the gate electrode the N+ type region 104.

The present invention is based on the finding that an increase of the yield field strength of the voltage withstanding layer will drastically reduce the loss and heat generation of vehicular AC generators, on the basis of the above-discussed analyses: that it is difficult to achieve desirable MOS power transistors for vehicular AC generators using conventional silicon MOS power transistors; that it is essential to drastically reduce the resistance of the voltage withstanding layer in order to achieve a MOS power transistor three-phase full wave rectifier; that it is essential to drastically reduce the thickness of the voltage withstanding layer and drastically increase the impurity concentration of the layer for the drastic reduction of the resistance of the layer; and that the drastic reduction of the thickness of the voltage withstanding layer and the increase of the impurity concentration of the layer is possible only after the yield field strength of the voltage withstanding layer is drastically increased. Accordingly, another object of this invention is to provide a vehicular AC generator having a small-size rectifier that can be disposed in a high-temperature site without requiring a large installation space, by mounting a rectifier that achieves a drastic loss reduction, and is easy to cool and highly heat resistant compared with a conventional rectifier, to the housing.

A very compact and easy-to-wire construction of a rectifier can be achieved by using vertical MOS power transistors having a vertical channel structure as the above-mentioned MOS power transistors, and joining the substrate of the vertical MOS power transistor forming the high side switch of the rectifier to a metal substrate (or a cooling fin) connected to the battery lower potential terminal. However, this construction encounters a troublesome problem in that a large current flows though a parasitic diode in the low side switch vertical MOS power transistor because of the short circuit between the n+ surface region and the P well region conventionally performed to set the electric potential of the P well region.

Accordingly, still another object of this invention is to provide a vehicular AC generator having a full wave rectifier comprising MOS power transistors that enable the potential setting of the P well regions while substantially preventing parasitic current.

The above objects are achieved in a first aspect of the present invention by providing a vehicular AC generator including a housing fixed to a transversely-mounted engine whose exhaust pipe is disposed in front with respect to the vehicle and a rectifier having a semiconductor rectifying device fixed to the housing, the vehicular AC generator being belt-driven by the engine, the vehicular AC generator being characterized in that the semiconductor rectifying device includes a MOS transistor whose substrate is formed of monocrystalline SiC and which includes a given conduction type semiconductor region having an inverted channel formed in a surface region thereof and an opposite conduction type source region and drain region that are connected to each other by the inverted channel, and that the rectifier is fixed to an exhaust pipe-side end wall of the housing.

In this arrangement, the rectifier employing SiC MOS power transistors as semiconductor rectifying devices is fixed to the exhaust pipe-side end wall of the generator housing, that is, the end wall remote from the pulley.

Since a large amount of magnetic energy is accumulated in the three-phase armature winding of a vehicular AC generator as mentioned above, each semiconductor device of the three-phase full wave rectifier needs to withstand a voltage, for example, about 300 V, at least twenty times the battery voltage, that is, the output voltage of the three-phase full wave rectifier, as a measure for instant release of the accumulated energy. In addition, a current as great as or greater than 100 A is required as the maximum output current for recently increased vehicle-installed electric loads.

The yield field strength of SiC is approximately 400 V/(m, that is, approximately thirteen times as great as that of Si.

The much greater yield field strength of SiC compared with that of Si offers an advantage of drastically reducing the power loss of MOS power transistors when used as a component device of a vehicular AC generator. The power loss reducing effect is produced on the basis of the difference in yield field strength between the SiC and Si.

An example case where a 300 V voltage withstanding characteristic is achieved by using SiC MOS power transistors in a vehicular AC generator shown in FIG. 3 will be considered. To simplify the discussion, it is assumed that the N type voltage withstanding layer 105 (see, for example, FIG. 4) solely bears 300 V.Assuming that the N type voltage withstanding layer 105 bears the voltage of 300 V and that the yield field strength of SiC is 400 V/pm, the required thickness of the N type voltage withstanding layer 105 becomes about 4 pm, the impurity concentration becomes 2 x 1016 atoms/cm3, the resistivity becomes approximately 1.25 Docs. On the other hand, as for the 300 V voltage withstanding layer of the Si-MOS power transistor described above, its required thickness is approximately 20 pm, the impurity concentration is 1 x 1015 atoms/cm3, the resistivity is approximately 5 Qcm. Thus, the resistance of the N type voltage withstanding layer 105 of a SiC MOS power transistor is reduced to 1/20 of the resistance of the N type voltage withstanding layer 105 of a Si-MOS power transistor.The impurity concentration of the N type voltage withstanding layer 105 could be reduced to a value much less than the above-mentioned value, depending on the relation with the impurity concentration of the P type well region 103.

Thus, the vehicular AC generator employing SiC MOS power transistors according to the invention drastically reduces the resistant power loss caused by the voltage withstanding layer, that is, the source parasitic resistance RS, and, furthermore, achieves a drastic reduction of the channel resistance by reducing the above-mentioned source resistance feedback effect.

With the combined reduction, the vehicular AC generator employing SiC MOS power transistors achieves a very low power loss compared with a vehicular AC generator employing Si-MOS power transistors or a diode three-phase full wave rectifier having substantially the same power loss as the Si-MOS power transistor. As an additional advantage, the vehicular AC generator of the invention is very easy to cool.

Since the three-phase full wave rectifier employing SiC-MOS power transistors as semiconductor devices achieves a very low power loss and a high heat resistance compared with conventional vehicular three-phase full wave rectifiers, the cooling structures, such as a heat sink or cooling fins, can be accordingly simplified and reduced in size and, in addition, the required cooling air flow and cooling air current temperature and other required conditions can be made much easier to achieve than in the conventional art.

With the reduced heat generation and the improved heat resistance, the rectifier having SiC-MOS power transistors can be fixed to the exhaust pipe-side end wall of the vehicular AC generator fixed to the front side of a transversely-mounted engine with the front exhaust pipe layout without requiring a large cooling fin or a special heat blocking structure for reducing heat radiation from the exhaust pipe.Thus, the vehicular AC generator of the invention achieves various advantages, such as size reduction of the generator due to the size reduction of the rectifier cooling fins, enhancement of the cooling effect on the armature winding and the field winding, avoidance of complicated belt installing steps, enhancement of the cooling effect on the generator by natural cooling currents caused by the traveling of the vehicle and by forced cooling currents generated by the cooling fan, reduction of the required engine room space for the engine and the generator, and omission of the heat blocking structure that is conventionally provided between the rectifier and the exhaust pipe.

The vehicular AC generator may further include an end cover which is fixed to an outer end surface of the exhaust pipe-side end wall of the housing to cover the rectifier fixed to the outer end surface of the exhaust pipe-side end wall, and which is disposed so that an outer surface of the end cover faces the exhaust pipe. In this way, the end cover faces the exhaust pipe and receives the heat radiated therefrom, without a heat blocking member disposed therebetween. Thus, this construction avoids the difficulty in disposing a heat blocking member in a narrow gap between the end cover and the engine without allowing the member to contact either the end cover or the engine (for prevention of heat conduction), and the complication of maintenance of the generator and other auxiliary devices, while reducing the number of required component parts and assembly man-hours.

The rectifier may be disposed in a package which also accommodates a gate controller for controlling the turning on and off of the MOS transistor and a regulator for maintaining battery voltage at a reference level. Thus, since the heat generation of the rectifier is small according, it is possible to mount the rectifier chips and the IC chips constituting the gate controller and the regulator on a single substrate. The thus-achieved single hybrid IC chip arrangement will remarkably simplifies the wiring processes.

Also, the MOS transistor may include a high side switch for connecting each output terminal of the armature winding of the generator separately to the higher potential terminal of a battery, and a low side switch for connecting each output terminal of the armature winding of the generator separately to the lower potential terminal of the battery, where the high side switch includes a vertical MOS power transistor in which a main electrode connecting to the higher potential terminal of the battery is formed from an n+ substrate and a P well region is connected to the main electrode connected to the armature winding, and the low side switch includes a vertical MOS power transistor in which a main electrode connected to the lower potential terminal of the battery is formed from an n+ substrate and a P well region is connected to the main electrode connected to the armature winding through a current-limiting high resistance. In this way, the high-side and low-side switches of the rectifier are formed in a vertical MOS power transistor construction, and their n+ substrates are used as the main electrodes connecting to the higher and lower potential terminals of the battery.

This construction makes it possible to directly fix the substrates of the MOS power transistors forming the low side switches to a common metal substrate (a ground-side cooling fin) connected to the battery lower potential terminal and to directly fix the substrates of the MOS power transistors forming the high side switches to a common metal substrate (a B terminal-side cooling fin) directly connected to the battery higher potential terminal, thus considerably simplifying the construction of the rectifier.

In a preferred construction, each high side switch of the three-phase full wave rectifier can be formed from a single semiconductor chip, and each low side switch of the three-phase full wave rectifier can be formed from a single semiconductor chip. The current-limiting high resistance of each low side switch is employed to set the electric potential of the P well region of the MOS power transistor forming the low side switch.

In vertical MOS power transistors, the potential setting of the P well region needs to be established from the surface n+ region.

This is because a sufficient voltage resistance needs to be achieved by the n- voltage withstanding layer formed between the n+ substrate and the P well region.

According to another aspect of the present invention, the above objects are achieved by providing a vehicular AC generator including a high side switch for connecting each output terminal of an armature winding separately to the higher potential terminal of a battery, and a low side switch for connecting each output terminal of the armature winding separately to the lower potential terminal of the battery, where the high side switch includes a vertical MOS power transistor in which a P well region is connected to an n+ surface region that forms a main electrode connected to the armature winding and the n+ substrate is joined to a cooling fin closer to a B terminal connected to the higher potential terminal of the battery, and that the low side switch includes a vertical MOS power transistor in which a P well region is connected to an n+ surface region that forms a main electrode connected to the armature winding through a high resistance and the n+ substrate is connected to a ground-side cooling fin connected to the lower potential terminal of the battery.

In this way, it is possible to set the electrical potential of the P well region of the high-side and low-side switches, and to directly mount the n+ substrate of the vertical MOS power transistors forming the high side switches to the B terminal-side cooling fin, and to directly mount the n+ substrate of the vertical MOS power transistors forming the low side switches to the ground-side cooling fin. Thus, the wiring structure and overall construction of the rectifier can be considerably simplified and reduced in size, and the cooling effect will also be improved.

The high side switch may be integrated in a first semiconductor chip, and the low side switch may be integrated in a second semiconductor chip. Thus, since the high side switches are integrated in a first semiconductor chip and the low side switches are integrated in a second semiconductor chip, the rectifier construction will become more simplified.

The ground-side cooling fin may be affixed to the housing of the vehicular AC generator, and the B terminal-side cooling fin may be affixed to the ground-side cooling fin with an insulation film disposed therebetween. This construction reduces the generator's size and simplifies its wiring while improving its conductive cooling performance.

Other objects and features of the invention will appear in the course of the description thereof, which follows.

Additional objects and advantages of the present invention will be more readily apparent from the following detailed description of preferred embodiments thereof when taken together with the accompanying drawings in which: FIG. 1 shows the arrangement of an engine with a vehicular AC generator according to a preferred embodiment of the present invention; FIG. 2 is a sectional view of the vehicular AC generator shown in FIG. 1; FIG. 3 is a circuit diagram of the vehicular AC generator shown in FIG. 1; FIG. 4 is an equivalent circuit diagram of an inverter circuit, illustrating a single-phase portion of the three-phase full wave rectifier shown in FIG. 1; FIG. 5 is a enlarged partial sectional view of an example of the MOS power transistors forming the high side switches of the three-phase full wave rectifier shown in FIG. 1;; FIG. 6 is a enlarged partial sectional view of an example of the MOS power transistors forming the low side switches of the three-phase full wave rectifier shown in FIG. 1; FIG. 7 is a graph indicating the voltage-current characteristic of a conventional PN diode formed of Si; FIG. 8 is a graph indicating the voltage-current characteristic of a conventional Si-MOS power transistor; FIG. 9 is a graph indicating the voltage-current characteristic of a SiC-MOS power transistor according to the embodiment; FIG. 10 is a graph indicating the relationship between the withstanding voltage of the MOS power transistors shown in FIGS.

7 and 8 and the on-resistivity when the devices are energized; FIG. 11 is a graph indicating the relationships between the rotational speed and the output current and between the rotational speed and the efficiency of vehicular AC generators employing a Si-MOS power transistor three-phase full wave rectifier or a SiC-MOS power transistor three-phase full wave rectifier; FIG. 12 is a partial plan view of the voltage regulator; FIG. 13 is a longitudinal sectional view of the voltage regulator shown in FIG. 12; FIG. 14 illustrates an arrangement of the voltage regulator according to a modification of the present invention; FIG. 15 illustrates the configuration of the voltage regulator shown in FIG. 14; FIG. 16 is a sectional view of a single chip in which the high side switches are integrated according to a second preferred embodiment of the present invention; and FIG. 17 is a sectional view of a single chip in which the low side switches are integrated according to he second embodiment.

An arrangement of a vehicular engine and a vehicular AC generator according to a first preferred embodiment of the present invention is shown in FIG. 1. The overall construction of the vehicular AC generator will be described with reference to FIG. 2.

A transversely-mounted engine 100 with the front exhaust pipe layout has an exhaust pipe 101 extending toward the front of the vehicle. The engine arrangement is cooled by cooling air currents from a cooling fan (not shown) and the ventilation caused by the traveling of the vehicle. A vehicular AC generator 200 has a pulley 201 fixed to the rotational shaft of the generator. The pulley 201 is driven by the engine 100 through a belt 202. A voltage regulator 18 is fixed to an end wall of the vehicular AC generator remote from the pulley 201 (the exhaust pipe-side end wall) and is covered with an end cover 23.

A housing of the generator 100 includes a drive frame 1 and a rear frame 2 that are formed by aluminum die casting and coupled by a plurality of stud bolts (not shown). The end cover 23 formed of an aluminum plate is fixed to a rear end wall 2a of the rear frame 2 (the end wall remote from the pulley 201 or the exhaust pipe-side end wall). Fixed to the outer surface of the rear end wall 2a is the voltage regulator 18 covered with the end cover 23. The end cover 23 is disposed near the exhaust pipe 101, and receives radiant heat from the exhaust pipe 101.

Stator cores 3 are fixed to the inner peripheral surface of the frame 1, and are provided with three-phase armature windings 5. Bearings 13, 14 fixed to the frames 1, 2 rotatably support a shaft 9. A rotor core 6 is fixed to the shaft 9 and faces the inner peripheral surfaces of the stator cores 3. The rotor core 6 is provided with a field winding 10. Cooling fans 11, 12 are disposed near opposite end surfaces of the rotor core 6. Cooling air inlet apertures W are formed in the end walls of the drive frame 1 and the rear frame 2. Cooling air outlet apertures W' are provided in the peripheral walls of the drive frame 1 and the rear frame 2.

The circuit configuration of the vehicular AC generator according to this embodiment will be described with reference to FIG. 3. The voltage regulator 18 includes a three-phase full wave rectifier (rectifier) 19 and a controller (serving as a gate controller and a regulator) 20 that generates gate control voltages for the on-off control of MOS power transistors l9a 19f of the rectifier 19 and controls the turning on and off of field current to a field coil 10.

The rectifier 19 is a three-phase full wave rectifier including the N channel enhancement-type MOS power transistors 19a - 19f formed of monocrystalline SiC. The high side transistors 19a - 19c connect the individual phase output terminals of the three-phase circuit armature windings 5 to the higher potential terminal of a battery 21. The low side transistors 19d - 19f connect the individual phase output terminals of the three-phase circuit armature windings 5 to the lower potential terminal of the battery 21.

The voltage regulator 18 is connected to the field winding 10 by a brush 16 and a slip ring 17. The voltage regulator 18 receives individual phase voltages from the individual phase output terminals of the three-phase armature windings 5. Based on these input signals, the voltage regulator 18 controls the gate voltage that is applied to the respective gate electrodes of the MOS power transistors 19a - 19f.

The voltage control operation will be briefly described.

When the rotor core 6 is rotated by the engine (not shown) and the voltage regulator 18 detects the voltage of battery 21 and controls the turning on and off of the field windings 10 so as to maintain the voltage at a constant level, three-phase voltages are induced across the three-phase armature windings 5, and the direct currents full wave-rectified by the three-phase full wave rectifier 19 charges the battery 21 and are consumed by vehicle-installed electronic loads and the like. The cooling fans 11, 12 are operated to cool the field winding 10, the three-phase armature windings 5 and the voltage regulator 18.

The on-off control of the MOS power transistors 19a - l9f of the three-phase full wave rectifier 19 by the controller 20 will be described.

The controller 20 receives the generated voltages Vu, Vv, Vw, that is, the potentials of the individual phase output terminals of the three-phase armature windings 5, and selects one of the interphase generated voltages Vu-Vv, Vv-Vw, Vw-Vu that is higher than the terminal voltage of the battery 21, and turns on one of the high side MOS power transistors 19a - 19c and one of the low side MOS power transistors 19d - l9f to apply the selected interphase generated voltage to the battery 21. Thus, charging current is fed to the battery 21 from the selected three-phase armature windings.

The controller 20 detects the terminal voltage of the battery 21 and compares the detected voltage with a predetermined reference voltage and controls the turning on and off of the exciting current on the basis of the voltage level comparison to maintain the terminal voltage of the battery 21 at a target level.

The three-phase full wave rectifier including SiC-MOS power transistors (also referred to herein as "MOS transistors") will be further described with reference to FIGS. 4 and 5. FIG. 4 shows an inverter circuit illustrating a single phase portion of the MOS power transistor type three-phase full wave rectifier according to this embodiment. FIG. 5 shows an example cross-sectional construction of any one of the MOS power transistors 19a - 19c. FIG. 6 shows an example cross-sectional construction of any one of the MOS power transistors 19d - 19f.

In the inverter circuit comprising N channel MOS power transistors as shown in FIG. 4, a drain electrode D of a high-side MOS power transistor 101 and a source electrode S of a low-side MOS power transistor 102 are connected to a single-phase output terminal of the three-phase armature windings 5. A drain electrode D of the low-side MOS power transistor 102 is connected to the lower potential terminal of the battery 21.

A source electrode S of the high-side MOS power transistor 101 is connected to the higher potential terminal of the battery 21.

The direction of electron transfer and the direction of charging current during charging are opposite one another. The source electrodes S are the electrodes for injecting carrier charges into the channels during charging.

In each of the MOS power transistors 101, 102, a source-side parasitic diode Ds and a drain-side parasitic diode Dd are formed between a P type well region 103 (that is, a region immediately beneath the gate electrode G) and a source electrode S and between the P type well region and the drain electrode D as indicated in FIG. 4. In the MOS power transistor 101, the P type well region 103 is short-circuited to the drain electrode D in order to provide an electric potential for the P type well region 103. Thereby, the source-side parasitic diodes Ds prevents the reverse current from the battery 21.

In the MOS power transistor 102, the P type well region 103 is connected to the source electrode S through a current-limiting high resistance r in order to provide an electric potential for the P type well region 103.

The example cross-sectional construction of the MOS power transistors 101 forming the high side switches 19a - 19c according to this embodiment will be described with reference to FIG. 5. An N type voltage withstanding layer 105 is formed on a SiC N+ type substrate 106 by epitaxial growth. The P type well region 103 is formed in a surface portion of the N type voltage withstanding layer 105 by epitaxial growth. Furthermore, an N+ type region 104 is formed in a surface portion of the P type well region 103 by nitrogen ion implantation. Then, the wafer surface is masked with a resist or an insulation film that exposes only a region to be formed into a trench, and a trench 108 is formed by well-known reactive ion etching (RIE) dry etching. A gate insulation film 109 of silicon oxide is then formed on the surface of the trench 108 by a thermal oxidation method.After that, a gate electrode 110 of doped polysilicon is formed on the trench 108. Then, a metal electrode 111 is disposed on the surfaces of the N+ type region (drain electrode) 104 and the P type well region 103, and a metal electrode 11 is disposed on the surface of the N+ type substrate (source electrode) 106 to complete the device.

An example cross-sectional construction of the MOS power transistors 102 forming the low side switches 19d - 19f according to this embodiment will be described with reference to FIG. 6.

In the construction shown in FIG. 6, the short circuit between the drain D and the P well region 103 is eliminated, and the P well region 103 is connected to the source S through the high resistance r as indicated in FIG. 4. Thus, while the source S of each of the high side switches l9a - 19c is formed by the substrate 106 as shown in FIG. 5, the source S of each of the low side switches 19d - 19f is formed by an n+ surface region 104 as shown in FIG. 6.

According to this embodiment, when a high voltage (for example, +300 V) is applied between the source electrode 106 and the drain electrode 111 while the MOS power transistor 101 is off, a depletion layer expands into the N type voltage withstanding layer 105. As a result, the N type voltage withstanding layer 105 acts as a source feedback resistor Rs, thus producing power loss due to both its own resistance and the channel resistance increase effect as mentioned above. However, since the MOS power transistor is formed of monocrystalline SiC according to this embodiment, the thickness and the impurity concentration of the N type voltage withstanding layer 105 can be improved to desirable states compared with conventional Si-MOS power transistors.

Setting conditions of the N type voltage withstanding layer 105 will be discussed on assumption that the N type voltage withstanding layer 105 has the 300 V withstanding characteristic.

The yield field strength of Si is approximately 30 V/pm.

Assuming that 300V is borne by the N type voltage withstanding layer 107 to simplify the discussion, the required thickness of the voltage withstanding layer becomes approximately 20 pm, the impurity concentration becomes 1 x 1015 atoms/cm3, and the resistivity becomes approximately 5 Dcm.

On the other hand, the required thickness of the SiC N type voltage withstanding layer 105 becomes approximately 4 pm, the impurity concentration becomes 2 x 1016 atoms/cm3, and the resistivity becomes approximately 1.25 Dcm, assuming that the yield field strength of SiC is 400 V/pm. Thus, the resistance of the N type voltage withstanding layer 107 of the SiC-MOS power transistor can be reduced to 1/20 of the resistance of the N type voltage withstanding layer 107 of the Si-MOS power transistor.

According to this embodiment, the source parasitic resistance Rs in the SiC-MOS power transistor can be reduced to 1/20 of that of Si-MOS power transistor and, accordingly, the channel resistance can be greatly reduced as stated above. With the combined reduction, this embodiment provides a very low power loss three-phase full wave rectifier for a vehicular AC generator.

The above description has made it clear that employment of SiC-MOS power transistors enhances the yield field strength of the N type voltage withstanding layer 105 and makes it possible to provide a highly efficient three-phase full wave rectifier 19 that could not be achieved according to the conventional art.

It should be understood that the above-discussed relationship holds if a high voltage other than 300V is applied to the N type voltage withstanding layer 105. FIGS. 7 to 9 indicate the voltage-current characteristics of a Si diode, a Si-MOS power transistor and a SiC-MOS power transistor that were produced on the same chip size by the same design rules. These devices withstand 250 V. FIG. 7 indicates the characteristic of the Si diode. FIG. 8 indicates the characteristic of the Si-MOS power transistor. FIG. 9 indicates experimental results of the SiC-MOS power transistor. As understood from FIGS. 7-9, the three-phase full wave rectifier 19 according to this embodiment reduces the power loss (heat generation) by 90% or more at an output current of 75 A, compared with the conventional three-phase full wave rectifiers.

FIG. 10 indicates example results of calculation of the on-resistivities for various required voltage withstanding characteristics of the MOS power transistor. The on-resistivity is the sum of the channel resistance and the resistance of the N type voltage withstanding layer 105. Although the channel resistance, in particular, varies depending on various factors, the resistance of the N type voltage withstanding layer 105 becomes dominant in a high withstanding voltage region as indicated in FIG. 10.

More specifically, while the channel resistance remains substantially unchanged as the withstanding voltage increases (if the increase of the channel resistance caused by the above-mentioned feedback effect due to an increase of the source parasitic resistance Rs is ignored),. the resistance of the N type voltage withstanding layer 105 increases while holding a positive correlation with the withstanding voltage. As indicated, the on-resistivity of the Si-MOS power transistor increases proportional to the withstanding voltage of about 25 V and higher. As for the SiC-MOS power transistor, on the other hand, the resistance increase of the N type voltage withstanding layer 105 is negligible up to a withstanding voltage of 250 V, and the on-resistivity starts to slowly increase as the withstanding voltage exceeds 250 V.

FIG. 11 indicates the characteristics of the vehicular AC generator employing a three-phase full wave rectifier 19 incorporating SiC-MOS power transistors and Si-MOS power transistor of equal chip size (control example) for additional illustration of this embodiment. To set consistent conditions for comparison with the conventional art, the three-phase full wave rectifier 19 was fixed to the outside surface of the rear frame. The output current loss was increased by approximately 10% (with 12 poles, at 5000 rpm). Because the rectification loss was substantially negligible, the rectifying efficiency was improved by approximately 3-5%.

The relationship between the withstanding voltage and the resistance of the Si-MOS and SiC-MOS power transistors will be described below.

The MOS power transistors l9a - 19f according to the embodiments are formed of 6H-SiC to withstand 205 V. The analysis of the resistance values of the vehicular AC generator three-phase full wave rectifier 19 employing the 6H-SiC MOS power transistors l9a - l9f and a vehicular AC generator employing Si-MOS power transistors will be discussed. It is herein assumed that the channel resistance increase effect due to the feedback effect of the source parasitic resistance Rs is ignored. In addition, the circuits have a vertical structure as shown in FIGS. 5 and 6, and the chip areas are equal.If the transistor resistance R is the sum of the channel resistance rc and the resistance rb of the N+ type voltage withstanding layer 105, and rc = L/W. (l/s0Es.eo)1' (Tox/(Vg-Vt)) rb = 4Vb2.(1/ .es.eo.Ec.A) then, the resistance of the SiC-MOS power transistor becomes about 1/15 of the resistance of the Si-MOS power transistor. In the above mathematical expressions, the yield field strengths Ec of Si and SiC are 3x105 V/cm and 3x106 V/cm, respectively, the specific inductive capacities (s of Si and SiC are 11.8 and 10.0, the areas A are both 1 mm2; and Vb is break-down voltage (withstanding voltage).Furthermore, the electron bulk mobilities of Si and SiC are 1100 cm2/(V.S) and 370 cm2/(VS), respectively; the channel lengths L are both 1 pm; the channel widths W are both 222 pm; and the electron channel mobilities of Si and SiC are 500 cm2/(V.S) and 100 cm2/(VS), respectively.

As understood from the above expressions, SiC exhibits less resistance than Si at withstanding voltages of 50 V and higher.

The calculations based on the above expressions are performed on assumption that the substrates are used as drains. If the substrates are used as the sources, the resistance of Si should considerably increase by the channel resistance increase due to the above-mentioned feedback effect of the source parasitic resistance Rs. Therefore, it can be expected that the SiC-MOS power transistor will surely exhibit less resistance at the withstanding voltages of 100 V and higher at voltages exceeding 100 V even if the design rule is varied to some extent.

The construction and arrangement of the voltage regulator 18 according to this embodiment will be described hereinafter with reference to FIGS. 12 and 13. FIG. 12 is a partial plan view of the voltage regulator 18, illustrating the rectifier 19 in particular. FIG. 13 is a sectional view of the voltage regulator 18 shown in FIG. 12.

The voltage regulator 18 has a low side substrate (a ground-side cooling fin) 190 formed from an aluminum plate and a cover (a ground-side cooling fin) 191 that is formed from an aluminum plate and welded at its periphery to the low side substrate 190. A bolt 193 is inserted through a hole 192 formed in the low side substrate 190 and screwed into a threaded hole formed in the rear end wall 2a of the rear frame 2, thereby pressing the cover 191 against the rear end wall 2a. Thus, the heat received by the low side substrate 190 is efficiently conducted to the rear end wall 2a through the cover 191.

Joined to the inside surface of the low side substrate 190 are the n+ substrate 106 of the MOS power transistors 19d - 19f (see FIG. 6) forming the low side switches, an electrical insulation film 194 and alumina electric wiring substrates 195.

Required electric wiring 1950 is patterned on the alumina electric wiring substrates 195. For simplified illustration, the wiring substrates 195 are omitted from the drawing of FIG. 12.

A B-terminal-side cooling fin 1940 formed from an aluminum plate is adhered to the electrical insulation film 194. The n+ substrate 106 of the MOS power transistors 19a - 19c (see FIG.

5) forming the high side switches are adhered to the B terminal-side cooling fin 1940. A bus bar 196 connects contact electrodes formed in n+ surface regions of the MOS power transistors 19a, 19d to a U-phase output terminal of the three-phase armature winding 5. Contact portions of the bus bar 196 are plated with gold or tin-lead, and joined to the contact electrodes of the chips by a known method. Similarly, a bus bar 197 connects contact electrodes formed in n+ surface regions of the MOS power transistors l9b, 19e to a V-phase output terminal of the three-phase armature winding 5. A bus bar 198 connects contact electrodes formed in n+ surface regions of the MOS power transistors 19c, 19f to a W-phase output terminal of the three-phase armature winding 5.

A chip 20a carrying a Si bipolar integrated circuit that embodies the controller 20 is connected to the electric wiring 1950 by bonding wires 199. Bonding wires 1990 connect the electric wiring 1950 to contact regions 1991 connected to the gate electrodes 110 of the MOS power transistors 19a - 19f (see FIGS. 5, 6). Each of bus bars 196-198 is connected to a pin 1901 fixed to the substrate 190 by a hermetic seal 1902, and is thus connected to the outside, as shown in FIG. 13. Similarly, a bolt-like B terminal (not shown) is fixed to the substrate 190 by a hermetic seal and joined to the substrate 1940. Various signal terminals are connected to the outside by pins similar to the pin 1901. The resin film 194 sandwiched between the substrate 190 and the substrate 1940 may be replaced by a ceramic sheet having good thermal conductivity. If a ceramic sheet is used, it is advantageous to prepare a soldering layer or an electrically conductive adhesive layer. In other words, soldering may be used to join the substrate 190 to a chip (for example, the chip 19d), the substrate 190 to the ceramic sheet, the ceramic sheet to the substrate 1940, and the substrate 1940 to a chip (for example, the chip 19a).

According to the embodiment, since the heat generation from the three-phase full wave rectifier 19 is considerably reduced and SiC is highly heat resistant, it become possible to dispose the rectifier 19 near the exhaust pipe 101. In addition, since the heat generation from the rectifier 19 is reduced, the rectifier 19 and the controller 20 can be integrated in a single package, thus simplifying the electric wiring.

The end cover 23 that covers the voltage regulator 18 may be formed of a heat-resistant resin instead of a metal plate.

Since there is no need to provide a heat blocking structure between the exhaust pipe and the three-phase full wave rectifier 19, the construction and production process of the vehicular AC generator can be simplified. In addition, the rectifier 19 does not require a large cooling fin, while sufficient cooling of the three-phase armature winding 5 and the field coil 10 is achieved.

An example of the voltage regulator 18 is illustrated in FIGS. 14, 15.

FIG. 16 shows a sectional view of a single chip in which the MOS power transistors 19a - 19c according to a second preferred embodiment of the present invention are integrated. FIG. 17 shows a sectional view of a single chip in which the MOS power transistors 19d - 19f according to the second embodiment are integrated.

Deep trenches T1 reach a substrate 106, and shallow trenches T2 reach an n type voltage withstanding layer 105. A gate insulating film (not shown) is formed on a surface of each trench T2, and a gate electrode 110 is formed on each gate insulating film.

This construction will achieve further miniaturization.

Although the present invention has been fully described in connection with the preferred embodiment thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. Such changes and modifications are to be understood as being included within the scope of the present invention as defined by the appended claims.

Claims (20)

1. A vehicular AC generator comprising: a housing which can be fixed to an exhaust pipe take-out side of an engine; a rectifier having a semiconductor rectifying device; wherein said vehicular AC generator is for being belt-driven by said engine; said semiconductor rectifying device includes a MOS transistor unit having a substrate formed of monocrystalline SiC having a resistance smaller than that of Si; and said rectifier is fixed to an exhaust pipe-side end wall of said housing.

2. A vehicular AC generator according to claim 1, said vehicular AC generator further comprising an end cover which is fixed to an outer end surface of said exhaust pipe-side end wall of said housing to cover said rectifier fixed to said outer end surface of said exhaust pipe-side end wall, and which is disposed so that an outer surface of said end cover faces said exhaust pipe.

3. A vehicular AC generator according to claim 1 or 2, wherein said rectifier is disposed in a package which also accommodates a gate controller for controlling switching of said MOS transistor unit and a regulator for maintaining battery voltage at a reference level.

4. A vehicular AC generator according to any one of claims 1 to 3, wherein said MOS transistor unit comprises: a high side switch for connecting each output terminal of an armature winding of said generator separately to a first terminal of a battery; and a low side switch for connecting each output terminal of said armature winding of said generator separately to a second terminal of said battery, said second terminal having a lower potential than said first terminal; wherein said high side switch includes a vertical MOS power transistor in which a main electrode connected to said first terminal of said battery is formed from an n+ substrate and a P well region is connected to said main electrode connected to said armature winding; and said low side switch includes a vertical MOS power transistor in which a main electrode connected to said second terminal of said battery is formed from an n+ substrate and a P well region is connected to said main electrode connected to said armature winding through a current-limiting resistance.

5. A vehicular AC generator according to & ', ;rding claim, ierin: said MOS transistor unit is disposed in a sealed container; and said container is fixed to said housing of said vehicular AC generator and covered with an end cover fixed to said housing of said vehicular AC generator.

6. A vehicular AC generator according to claim 5, wherein: said housing of said vehicular AC generator has an inlet opening for introducing cooling air into said vehicular AC generator; and said inlet opening is formed in a portion of said housing other than a portion thereof for installing said container.

7. A vehicular AC generator according to claim 5 ar 6, wherein said container is a metal housing which is in heat-conductive contact with and fixed to said housing of said vehicular AC generator.

8. A vehicular AC generator according to claim 7, wherein: said MOS transistor unit comprises a high side switch and a low side switch; said metal housing forms a low side substrate; said low side switch is mounted on and electrically connected to an inside surface of said metal housing; a high side substrate is mounted on an electrically insulating member disposed on an inside surface of said metal housing; and said high side switch is mounted on and electrically connected to said high side substrate.

9. A vehicular AC generator according to claim 8, wherein said electrically insulating member is an electrically insulating film.

10. A vehicular AC generator according toclaim8cr9,wherin a voltage regulating unit for controlling output voltage from said vehicular AC generator is provided in said metal housing.

11. A vehicular AC generator aoOng to of S claims 5 to 10, hePin a rectifying unit including said MOS transistor unit and a voltage regulating unit for controlling output voltage from said vehicular AC generator are provided in said container.

12. A vehicular AC generator comprising: a high side switch for connecting each output terminal of an armature winding separately to a first terminal of a battery; and a low side switch for connecting each output terminal of said armature winding separately to a second terminal of said battery, said second terminal having a lower potential than said first terminal; wherein said high side switch includes a vertical MOS power transistor in which a P well region is connected to an n+ surface region that forms a main electrode connected to said armature winding and an n+ substrate is joined to a cooling fin closer to a B terminal connected to said first terminal of said battery; and said low side switch includes a vertical MOS power transistor in which a P well region is connected to an n+ surface region that forms a main electrode connected to said armature winding through a high resistance and an n+ substrate is connected to a ground-side cooling fin connected to said second terminal of said battery.

13. A vehicular AC generator according to claim 12, wherein: said high side switch is integrated in a first semiconductor chip; and said low side switch is integrated in a second semiconductor chip.

14. A vehicular AC generator according to claim 12 or 13, wherein: said ground-side cooling fin is affixed to said housing of said vehicular AC generator; and said B terminal-side cooling fin is affixed to said ground-side cooling fin with an insulation film disposed therebetween.

15. A rotational electrical apparatus mounted on and connected to an engine so that rotation can be transmitted from said engine to said apparatus, said rotational electrical apparatus being disposed near an exhaust pipe of said engine, said rotational electrical apparatus comprising: a fan for introducing cooling air into said rotational electrical apparatus; a sealed container disposed on a housing of said rotational electrical apparatus; and a semiconductor electrical device provided in said container, for controlling current through said rotational electrical apparatus; wherein said semiconductor electrical device is provided in said container and isolated from cooling air supplied from said fan.

16. A rotational electrical apparatus according to claim 15, wherein said container is fixed with respect to said housing of said rotational electrical apparatus so that heat conduction therebetween is possible.

17. A rotational electrical apparatus according to claim 16, wherein: said semiconductor electrical device provided in said container is a MOS transistor formed of SiC; and armature current of said rotational electrical apparatus is conducted through said semiconductor electrical device.

18. A rotational electrical apparatus according to claim 17, wherein: said rotational electrical apparatus is an AC generator, and said MOS transistor forms a rectifier that rectifies output current from said AC generator.

19. A vehicular AC generator substantially as described herein with reference to the accompanying drawing.

20. A rotational electrical apparatus substantially as described herein with reference to the accompanying drawings.